This invention relates to a heater for an indirectly-heated cathode for a cathode-ray tube (CRT) and more particularly to the structure of a heater for an indirectly-heated cathode which avoids damage to an insulating layer that covers the heater in a heater of a double helical structure.
As disclosed, for example, in Japanese Patent Laid-Open No. 12536/1984, heaters for an indirectly-heated cathode for CRTs generally have the structure wherein a core wire made of a refractory metal containing, for example, tungsten as its principal component is shaped in the form of a helical coil by a mandrel having a circular section.
FIG. 2 of the accompanying drawings illustrates a heater 1 for an indirectly-heated cathode having a double helical structure in accordance with the prior art. Reference numeral 2 represents a heater core wire made of a refractory metal containing tungsten as the principal component. As shown in FIG. 3, this heater core wire 2 itself may have a structure in the coil-like form and the heater core wire to be described hereinafter includes the core wires of this structure, also. Reference numeral 3 represents a known insulating layer which is formed on the heater core wire 2. An insulating layer made of alumina is deposited on the core wire 2 and there is formed thereon a layer consisting of a mixture of tungsten particles and alumina particles and having large thermal emissivity. Then, the layer is sintered in a hydrogen atmosphere at 1,650.degree. C., for example, to provide the insulating layer 3. Reference numeral 4 represents cracks of the insulating layer.
According to the prior art technique described above, the insulating layer 3 is likely to be damaged due to its brittleness. For example, when the heater 1 is put into or taken out from a container for transferring or when the legs of the heater 1 (the portions where the heater core wire 2 is exposed in FIGS. 2 and 3) are welded to a support portion of an electron gun structure, stress concentrates on the head of the heater (the opposite side to the legs) and cracks 4 are likely to develop in the insulating layer 3, as shown in FIG. 2. This stress is tensile stress due to impact or bending stress and torsional stress due to bending.
If such cracks 4 develop in the insulating layer 3 of the heater 1, the heater core wire 2 is exposed and when the heater 1 is incorporated in the cathode, insulation characteristics between the cathode heater 1 and the cathode 5 (including a cathode sleeve 21, a cap 22 and an electron emissive material 23 shown in FIG. 4 which is a sectional view of the principal portions) get deteriorated so that there occurs such problem that video signals of the CRT get distorted and picture quality drops. Furthermore, there also occurs the problem that particles falling off from the insulating layer 3 enter electron beam apertures of a shadow mask and clog them. This also results in a drop of picture quality. This tendency is all the more remarkable particularly in a high precision color picture tube having smaller electron beam apertures of the shadow mask than those of ordinary CRTs.